xe2x80x9cNot Applicablexe2x80x9d
xe2x80x9cNot Applicablexe2x80x9d
1. Field of the Invention
The invention pertains to a thermo-optical switch comprising a cascade of 1xc3x97M optical switches, gates for selectively blocking and unblocking the output paths of the cascaded switch, and means for driving the 1xc3x97M switches (M being an integer, preferably 2 or 3) and the gates, which means are arranged to switch a data signal from an input path of the cascaded switch to one of the output paths.
2. Description of Related Art
Such switches are known. WO 96/38756 discloses a thermo-optical switch with xe2x80x9cadditional branchesxe2x80x9d forming y-junctions with the actual output paths. As is explained in the description of said patent application, the additional branch is used to direct unwanted light away from the actual output path when this output is in the off-state. By doing so, the detrimental effect of the said light on both crosstalk and extinction (which are defined as 10*log(optical power in an output in the off-state/optical power in the input) and 10*log(optical power in an output in the on-state/optical power in an output in the off-state), respectively) is dispensed with before it reaches the output in the off-state.
WO 96/08932 describes a cascaded 1xc3x978 switch consisting of three switch stages of 1xc3x972 switches (1st stage: 1 switch, 2nd stage: 2 switches, and 3rd stage: 4 switches) and one shutter stage of 8 1xc3x972 switches, functioning as gates for selectively blocking and unblocking the output paths of the cascaded switch. The cascaded switch according to WO 96/08932 is also said to exhibit greatly improved crosstalk suppression.
In the known cascaded optical switches a signal at the input path is directed to the selected output path by operating the 1xc3x97M, usually 1xc3x972 switches, which are comprised in the cascaded or tree-structured switch. Since none of these 1xc3x97M switches has an infinite extinction, an unwanted signal is inevitably generated in each of the 1xc3x97M switches through which the data signal passes to the selected output. In the cascaded switches according to, e.g., WO 96/38756, detrimental effects of the unwanted signal can avoided by directing this signal away from the actual outputs by means of the gates.
In order to optimise extinction and crosstalk suppression, it is possible to activate all the gates in the last stage. Of course, the gates belonging to an output path which is in the on-state are used to direct the signal to the actual output path (the gate, in that case, is unblocking and in the off-state), whereas all the other gates in the last stage are activated to direct the unwanted signal away from the remaining outputs (these gates are blocking and in the on-state).
Now, in view of the present developments in the field of, amongst others, telecommunications, the requirements for thermo-optical cascaded switches are becoming ever more stringent. Reliability and life-expectancy should increase, the number of output paths (at present usually 8, in the future probably 16, 32 or 64) of the cascaded switches should also increase, and power consumption should decrease, all at an equal or, preferably, improved extinction ratio.
The object of the present invention is to meet said demand, and this is achieved in the thermo-optical cascaded switch described in the opening paragraph wherein the means for driving the 1xc3x97M switches and the gates are also arranged to switch an unwanted signal (generated by 1xc3x97M switch through which the data signal passes), by means of a number of the remaining 1xc3x97M switches (i.e., those switches which are not used to switch the data signal to the output in the on-state), to at least one of the remaining output paths (i.e., the output paths in the off-state) and to block these output paths.
Surprisingly, it was found that by using the remaining 1xc3x97M switches for switching unwanted signals towards one or more particular output paths and blocking the output path(s) in question by means of a gate, (most on the other gates need not be activated. As will be demonstrated below, the total number of 1xc3x97M switches and gates that must be activated can thus be reduced (dramatically) in cascaded thermo-optical switches having a large number of output paths and, consequently, the power consumption and the heat generated in the cascaded switch can be reduced considerably, which in turn leads to increased reliability and life-expectancy of the cascaded switch.
Further, unlike in the case where all the gates in the last stage are activated, the activated 1xc3x97M switches and gates are no longer concentrated in a small area, but instead distributed more evenly over the area of the cascaded switch. This is all the more advantageous since the gates are normally located near the pigtails (interconnections between the switch and the optical fibres), which are sensitive to high temperatures and temperature changes. In contrast, the 1xc3x97M switches are further removed from the pigtails and, hence, high temperatures and large temperature changes in these 1xc3x97M switches will have considerably less effect on the pigtails. Also, the even distribution of activated 1xc3x97M switches and gates will result in improved transportation of heat into the substrate on which the switch is built.
In a preferred embodiment the means for driving the 1xc3x97M switches and the gates are so arranged as to switch the unwanted signals generated in each of the 1xc3x97M switches through which the data signal passes to at least one of the remaining output paths by means of a number of the remaining 1xc3x97M switches. Thus, all first order crosstalk originating from the data signal is consistently (and actively) switched to a blocking gate.
As mentioned above, the number of activated heaters in cascaded switches consisting of 1xc3x972 switches and having 8, 16, 32, or 64 activated heaters is (considerably) reduced by using the drive tables according to the present invention. For drive tables activating all the gates the number of activated heaters is at least 11, 20, 37, and 70, respectively, whereas the drive tables of the above-mentioned preferred embodiment of the present invention allow activating only 10, 15, 21, and 28 heaters, i.e., a reduction of 1, 5, 16, and 42 activated heaters, respectively.